Acoustic device



June 4, 1935. T. SMITH EIAL ACOUSTIC DEVICE Filed April 25, 1934 L. T.SMITH MEMO/Q5 A. L. THU/PAS ATTORNEY Patented June 4/, 1935 UNITEDSTATES PATENT OFFICE Lee T. Smith, Madison, N. 1., and Albert L. Thuras,New York, N. Y., assignors to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application April 25, 1934,Serial No. 722,286

'7 Claims.

This invention relates to acoustic devices, and more particularly, todiaphragms for transmitters or microphones.

In a telephone transmitter or microphone, it

5 is desirable to have a diaphragm that is substantiallynon-hygroscopic, that will not be substantially affected by variationsin atmospheric temperatures, that is low in density and in mechanicalstiffness, that may be shaped to and retain a desired contour, and thatis flame-resistant.

An object of this invention is to satisfy the need for such a diaphragm.

A feature of this invention comprises a diaphragm of polymerized styrol.

A more complete understanding of this invention will be obtained fromthe detailed description which follows, read with reference to theappended drawing, wherein:

Fig. 1 is a cross-sectional view of a fragment of an acoustic deviceembodying this invention; and

Fig. 2 is a perspectiveview of a diaphragm embodying this invention,partly broken away and partly in section.

A diaphragm of polymerized styrol may be made by pressing the solidmaterial into shape; or, by first dissolving the solid material in asuitable solvent, for instance, benzene, toluene,

, ethylene dichloride, trichlorethylene, or the like, and the spraying,brushing, dipping, or spinning it onto a plane surface, allowing thesolvent to evaporate to leave the material in sheet form.. Polymerizedstyrol may be ob-,- tained on the market under the names of Meta-Styrene", Victron and Resoglaz.

When the spinning method is employed, the

' solution is poured on a revolving plane. surface,

, preferably glass, on which it spreads to a thickness dependent uponthe viscosity of the solution, the rate of rotation of the surface, andsurrounding atmospheric conditions. A continuous process similar to thatused in the manufacture of motion picture films from solutions ofcellulose nitrate or cellulose acetate may be used, also.

A diaphragm of a particular configuration, for instance, one having acentral stiffened portion, an outer peripheral portion, and anintermediate portion connecting the outer and the central portions andpermitting the central portion to vibrate substantially as a whole inresponse to impulses of sound wave frequency, may be obtained byoperating on the polymerized styrol sheet with suitably shaped dies.

The drawing shows a fragment of an electrodynamic type transmitter ormicrophone I0, comprising a casing II providing an enclosure for amagnet structure I2, which supports a diaphragm I3 embodying thisinvention.

The magnet structure comprises a center pole I4, for instance, of cobaltiron, having a domeshaped or cap member I5, for'instance, of magneticiron, secured thereto by any suitable means, for instance, bolt I 6, andan outer pole II, for instance, of cobalt steel. A plate pole I8,preferably annular, of magnetic iron and forming an annular air gap I9with the member I5, is supported on the outer pole, and has apcripheral, upwardly extending flange or ledge portion 20.

The diaphragm I3 is formed, preferably, from a sheet of polymerizedstyrol of substantially uniform thickness. A diaphragm as thin as .001to .0011" and of outside diameter of approximately 1" has been found togive satisfactory results. Such a diaphragm is substantiallynonhygroscopic and unaffected by atmospheric temperature variations, islow in densityand in mechanical stiffness, may be readily andpermanently shaped to a desired contour, and is flame-resistant. Thediaphragm comprises a central, stiffened, dome-shaped portion 2| ofshape similar to that of the upper surface 22 of the member I5; an outerperipheral and annular portion 23, adapted to be mounted on and securedby a suitable adhesive to the ledge portion surface 24; and anintermediate curved, outwardly concave portion 25, connecting the outerand central portions and being adapted to permit the portion 2I tovibrate substantially as a unit or piston, in response to impulses ofsound wave frequency. A coil 26 is attached to the underside of thediaphragm at the narrow, annular, planar'portion 21 connecting thecentral and intermediate portions, and is disposed in the air gap l9.Member 28 is a damping plate attached to the cap member by screws 29(one only being shown), and providing a narrow restricted passage 30connecting the air spaces or chambers 3|, 32 through the air gap IS withthe air space or chamber 33 in the magnet structure. Sound wave energyhas access to the diaphragm through the apertures 34 in the casing.

Although this invention has been described with reference to a specificembodiment thereof, it will be understood that it is capable of otherforms and sizes without departing from the spirit of the invention,which is to be considered as limited in scope, therefore, by theappended ciaims, only.

What claimed is:

1. An coustic device comprising a diaphragm of polymerized styrol.

2. An acoustic device comprising a diaphragm response to impulses ofsound wave frequency. 5. An acoustic device comprising a diaphragm ofpolymerized styrol of a thickness of about .001 to .0011".

6. An acoustic device comprising a diaphragm o1 polymerized styrolsubstantially 1 mil in thickness throughout, having a central stifienedportion, a peripheral mounting portion, and an intermediate portionconnecting the peripheral and central portions.

7. An acoustic device comprising a diaphragm of polymerized styrolhaving a central stiflened portion and being substantially 1 mil inthickness throughout.

LEE T. SMITH.

ALBERT L. THURAS.

